Three-knife trimmers for the three-sided cutting of stacks of books, brochures or the like are well known, the material to be cut being automatically brought to the cutting table, aligned there under the knives and held down on the cutting table by a pressure plate. The two side knives are immediately operated at the same time, in order to trim the head and foot of the stack, and then in a second work operation the front knife cuts the front of the stack, whereupon the cut material is automatically removed from the cutting table and leaves the machine by means of a conveyor belt.
This cutting sequence, which can also be reversed, is necessary because the side and front knives have to be longer than the material to be cut and therefore would be in each other's way in the case of simultaneous motion. In this regard, a familiar technique is to use a step-by-step switch, which controls the motions of the two side knives and the front knife in sequence.
Because of the time sequence of two work operations in cutting the material stack, the efficiency of the machine is naturally limited. The linking of machinery in book production lines requires, however, an increase in work tempo, since a three-knife trimmer of the type built up to now, when integrated into these high efficiency production lines, represents the slowest link in production and does not permit an optimal work tempo.
In contrast to three-knife trimmers with a single cutting station, two-station-flow line cutters are also known, which in contrast to the three-knife trimmer carry out the cut in two cutting stations, cutting the head and foot of the stack at the first station and the front in the following. Assembly line cutters are expensive, however, on account of their complicated construction, for additional measures must be taken to direct the stack of material being cut to the second station and to remove it from the latter, whereby the accuracy of the cut can suffer from the necessity of repositioning the stack. Besides, the problem of transportation limts the height of the stack, so that the advantage of increased pace must be purchased at a cost of decreased efficiency, so that no significant improvement of performance over the familiar three-knife trimmer can be achieved.
Further research in the direction of increased work tempo has thus concentrated on improvement of knife drive mechanisms in the three-knife trimmers.
DE-OS No. 28 26 476 has revealed the method of swinging the front knife away from the path of the side knives, after the former has completed its cut, so that the side knives can carry out their cuts immediately following.
It has been shown, however, that use of mechinery in high efficiency production lines requires considerable operating speeds, so that with the considerable masses which must be accelerated and decelerated, a speed limit is soon reached, if the motions are not harmonic. This is the case, however, when immediately following a cutting movement a very quick swinging movement of the knife is carried out, which runs in a different direction from the cutting movement. Besides, the knife is inclined to bend during the cut because of its swivel mounting.
From DE-PS No. 19 63 861 a three-knife trimmer is familiar, in which the movements of the knives are derived from the so-called single-revolution shaft of the machine. The movements of the side knives and the front knife come about one after the other, as before, during a revolution of the single-revolution shaft; however, the mechanism between the single-revolution shaft and the knife holders is so designed that not only the side knives but also the front knife remain for a period of time above the area of cut, so that this period of time is available for the exact positioning of the stack of material being cut.
The drive mechanism for the knife holders consists in this case of a series arrangement of guide unit, which guides the knife, and operating unit, which precedes the other. The guide unit raises and lowers the attached knife assembly, while the operating unit controls the time of operation of the guide unit according to a given function. The units consist of a chain of ten gear elements each. The numerous elements, however, lead to a high total tolerance of bearing play and a high elasticity of the entire chain, which can have a bad effect on the movement of the knives, e.g. at bottom dead center at the conclusion of the cut and the simultaneous reversal of the motion, when the long chain, having been strained under the force of the cut, relaxes again.
The invention is based on a three-knife trimmer of the last-named type, in which first the side knives and then the front knife, or vice versa, are operated to carry out the cut by a single-revolution shaft, during one revolution, by means of intermediate gears at a single location, whereby the drive system for the knives consists of a guide unit and an operating unit preceding it.